Illumination systems which comprise a reflector for reflecting light emitted by a light source towards a light-exit window of the illumination system are known per se. They are used, inter alia, as illumination systems of (image) display devices, for example in projection systems. Such illumination systems can particularly suitably be used as an illumination source in a digital projector, so-called beamer, for projecting images or displaying a television program, a film, a video program or a DVD, or the like. In addition, such illumination systems are used for general lighting purposes, such as for large-area direct-view light emitting panels such as applied, for instance, in signage, contour lighting, and billboards. The widespread use of light emitting diodes has also made this illumination system suitable for general non-emissive displays, such as liquid crystal display devices, also referred to as LCD panels, which are used in, for example, television receivers, (computer) monitors, (cordless) telephones and portable digital assistants. In the known backlight system of a LCD panel, typically an array of the illumination systems is used over the LCD panel.
The reflector in the illumination system is, generally, in the shape of a cup which substantially surrounds the light source apart from the light-exit window. Part of the light emitted by the light sources is directly incident on the light-exit window, other parts of the light emitted by the light sources is reflected by the reflector towards the light-exit window. In general, the reflector is made of a thin sheet of a specular reflective material. The sheet is shaped by means of bending or curving or otherwise.
U.S. Pat. No. 5,607,229 discloses an illumination system for illuminating non-circular apertures, which are commonly rectangular. The illumination system comprises a reflector of substantially ellipsoidal form surrounding a light source. The reflector has a concave reflection surface formed of a plurality of curved reflective segments extending along the length of the reflection surface. Each of the reflective segments is tilted and rotated by a predetermined amount to direct light from the reflector almost entirely into the area encompassed by the rectangular aperture including portions of the area which lie outside of a circular area inscribed within the aperture.
A drawback of the known illumination system is that the distribution of light over the rectangular light-exit window is not sufficiently uniform.